Carbohydrate ingestion and muscle glycogen depletion during marathon and ultramarathon racing

Limits of Ultra: Towards an Interdisciplinary Understanding of Ultra-Endurance Running Performance

Ultra-endurance running (UER) poses extreme mental and physical challenges that present many barriers to completion, let alone performance. Despite these challenges, participation in UER events continues to increase. With the relative paucity of research into UER training and racing compared with traditional endurance running distance (e.g., marathon), it follows that there are sizable improvements still to be made in UER if the limitations of the sport are sufficiently understood. The purpose of this review is to summarise our current understanding of the major limitations in UER. We begin with an evolutionary perspective that provides the critical background for understanding how our capacities, abilities and limitations have come to be. Although we show that humans display evolutionary adaptations that may bestow an advantage for covering large distances on a daily basis, these often far exceed the levels of our ancestors, which exposes relative limitations. From that framework, we explore the physiological and psychological systems required for running UER events. In each system, the factors that limit performance are highlighted and some guidance for practitioners and future research are shared. Examined systems include thermoregulation, oxygen delivery and utilisation, running economy and biomechanics, fatigue, the digestive system, nutritional and psychological strategies. We show that minimising the cost of running, damage to lower limb tissue and muscle fatigability may become crucial in UER events. Maintaining a sustainable core body temperature is critical to performance, and an even pacing strategy, strategic heat acclimation and individually calculated hydration all contribute to sustained performance. Gastrointestinal issues affect almost every UER participant and can be due to a variety of factors. We present nutritional strategies for different event lengths and types, such as personalised and evidence-based approaches for varying types of carbohydrate, protein and fat intake in fluid or solid form, and how to avoid flavour fatigue. Psychology plays a vital role in UER performance, and we highlight the need to be able to cope with complex situations, and that specific long and short-term goal setting improves performance. Fatigue in UER is multi-factorial, both physical and mental, and the perceived effort or level of fatigue have a major impact on the ability to continue at a given pace. Understanding the complex interplay of these limitations will help prepare UER competitors for the different scenarios they are likely to face. Therefore, this review takes an interdisciplinary approach to synthesising and illuminating limitations in UER performance to assist practitioners and scientists in making informed decisions in practice and applicable research.

What Is the Evidence That Dietary Macronutrient Composition Influences Exercise Performance? A Narrative Review

The introduction of the needle muscle biopsy technique in the 1960s allowed muscle tissue to be sampled from exercising humans for the first time. The finding that muscle glycogen content reached low levels at exhaustion suggested that the metabolic cause of fatigue during prolonged exercise had been discovered. A special pre-exercise diet that maximized pre-exercise muscle glycogen storage also increased time to fatigue during prolonged exercise. The logical conclusion was that the athlete's pre-exercise muscle glycogen content is the single most important acutely modifiable determinant of endurance capacity. Muscle biochemists proposed that skeletal muscle has an obligatory dependence on high rates of muscle glycogen/carbohydrate oxidation, especially during high intensity or prolonged exercise. Without this obligatory carbohydrate oxidation from muscle glycogen, optimum muscle metabolism cannot be sustained; fatigue develops and exercise performance is impaired. As plausible as this explanation may appear, it has never been proven. Here, I propose an alternate explanation. All the original studies overlooked one crucial finding, specifically that not only were muscle glycogen concentrations low at exhaustion in all trials, but hypoglycemia was also always present. Here, I provide the historical and modern evidence showing that the blood glucose concentration-reflecting the liver glycogen rather than the muscle glycogen content-is the homeostatically-regulated (protected) variable that drives the metabolic response to prolonged exercise. If this is so, nutritional interventions that enhance exercise performance, especially during prolonged exercise, will be those that assist the body in its efforts to maintain the blood glucose concentration within the normal range.

Neuromuscular responses to fatiguing locomotor exercise

Over the last two decades, an abundance of research has explored the impact of fatiguing locomotor exercise on the neuromuscular system. Neurostimulation techniques have been implemented prior to and following locomotor exercise tasks of a wide variety of intensities, durations, and modes. These techniques have allowed for the assessment of alterations occurring within the central nervous system and the muscle, while techniques such as transcranial magnetic stimulation and spinal electrical stimulation have permitted further segmentalization of locomotor exercise-induced changes along the motor pathway. To this end, the present review provides a comprehensive synopsis of the literature pertaining to neuromuscular responses to locomotor exercise. Sections of the review were divided to discuss neuromuscular responses to maximal, severe, heavy and moderate intensity, high-intensity intermittent exercise, and differences in neuromuscular responses between exercise modalities. During maximal and severe intensity exercise, alterations in neuromuscular function reside primarily within the muscle. Although post-exercise reductions in voluntary activation following maximal and severe intensity exercise are generally modest, several studies have observed alterations occurring at the cortical and/or spinal level. During prolonged heavy and moderate intensity exercise, impairments in contractile function are attenuated with respect to severe intensity exercise, but are still widely observed. While reductions in voluntary activation are greater during heavy and moderate intensity exercise, the specific alterations occurring within the central nervous system remain unclear. Further work utilizing stimulation techniques during exercise and integrating new and emerging techniques such as high-density electromyography is warranted to provide further insight into neuromuscular responses to locomotor exercise.

Physiology and Pathophysiology in Ultra-Marathon Running

In this overview, we summarize the findings of the literature with regards to physiology and pathophysiology of ultra-marathon running. The number of ultra-marathon races and the number of official finishers considerably increased in the last decades especially due to the increased number of female and age-group runners. A typical ultra-marathoner is male, married, well-educated, and ~45 years old. Female ultra-marathoners account for ~20% of the total number of finishers. Ultra-marathoners are older and have a larger weekly training volume, but run more slowly during training compared to marathoners. Previous experience (e.g., number of finishes in ultra-marathon races and personal best marathon time) is the most important predictor variable for a successful ultra-marathon performance followed by specific anthropometric (e.g., low body mass index, BMI, and low body fat) and training (e.g., high volume and running speed during training) characteristics. Women are slower than men, but the sex difference in performance decreased in recent years to ~10–20% depending upon the length of the ultra-marathon. The fastest ultra-marathon race times are generally achieved at the age of 35–45 years or older for both women and men, and the age of peak performance increases with increasing race distance or duration. An ultra-marathon leads to an energy deficit resulting in a reduction of both body fat and skeletal muscle mass. An ultra-marathon in combination with other risk factors, such as extreme weather conditions (either heat or cold) or the country where the race is held, can lead to exercise-associated hyponatremia. An ultra-marathon can also lead to changes in biomarkers indicating a pathological process in specific organs or organ systems such as skeletal muscles, heart, liver, kidney, immune and endocrine system. These changes are usually temporary, depending on intensity and duration of the performance, and usually normalize after the race. In longer ultra-marathons, ~50–60% of the participants experience musculoskeletal problems. The most common injuries in ultra-marathoners involve the lower limb, such as the ankle and the knee. An ultra-marathon can lead to an increase in creatine-kinase to values of 100,000–200,000 U/l depending upon the fitness level of the athlete and the length of the race. Furthermore, an ultra-marathon can lead to changes in the heart as shown by changes in cardiac biomarkers, electro- and echocardiography. Ultra-marathoners often suffer from digestive problems and gastrointestinal bleeding after an ultra-marathon is not uncommon. Liver enzymes can also considerably increase during an ultra-marathon. An ultra-marathon often leads to a temporary reduction in renal function. Ultra-marathoners often suffer from upper respiratory infections after an ultra-marathon. Considering the increased number of participants in ultra-marathons, the findings of the present review would have practical applications for a large number of sports scientists and sports medicine practitioners working in this field.

Untargeted Metabolomics Profiling of an 80.5 km Simulated Treadmill Ultramarathon

Metabolomic profiling of nine trained ultramarathon runners completing an 80.5 km self-paced treadmill-based time trial was carried out. Plasma samples were obtained from venous whole blood, collected at rest and on completion of the distance (post-80.5 km). The samples were analyzed by using high-resolution mass spectrometry in combination with both hydrophilic interaction (HILIC) and reversed phase (RP) chromatography. The extracted putatively identified features were modeled using Simca P 14.1 software (Umetrics, Umea, Sweden). A large number of amino acids decreased post-80.5 km and fatty acid metabolism was affected with an increase in the formation of medium-chain unsaturated and partially oxidized fatty acids and conjugates of fatty acids with carnitines. A possible explanation for the complex pattern of medium-chain and oxidized fatty acids formed is that the prolonged exercise provoked the proliferation of peroxisomes. The peroxisomes may provide a readily utilizable form of energy through formation of acetyl carnitine and other acyl carnitines for export to mitochondria in the muscles; and secondly may serve to regulate the levels of oxidized metabolites of long-chain fatty acids. This is the first study to provide evidence of the metabolic profile in response to prolonged ultramarathon running using an untargeted approach. The findings provide an insight to the effects of ultramarathon running on the metabolic specificities and alterations that may demonstrate cardio-protective effects.

The efficacy of protein supplementation during recovery from muscle-damaging concurrent exercise

This study investigated the effect of protein supplementation on recovery following muscle-damaging exercise, which was induced with a concurrent exercise design. Twenty-four well-trained male cyclists were randomised to 3 independent groups receiving 20 g protein hydrolysate, iso-caloric carbohydrate, or low-calorific placebo supplementation, per serve. Supplement serves were provided twice daily, from the onset of the muscle-damaging exercise, for a total of 4 days and in addition to a controlled diet (6 g·kg−1·day−1 carbohydrate, 1.2 g·kg−1·day−1 protein, remainder from fat). Following the concurrent exercise session at time-point 0 h, comprising a simulated high-intensity road cycling trial and 100 drop-jumps, recovery of outcome measures was assessed at 24, 48, and 72 h. The concurrent exercise protocol was deemed to have caused exercise-induced muscle damage (EIMD), owing to time effects (p < 0.001), confirming decrements in maximal voluntary contraction (peaking at 15% ± 10%) and countermovement jump performance (peaking at 8% ± 7%), along with increased muscle soreness, creatine kinase, and C-reactive protein concentrations. No group or interaction effects (p > 0.05) were observed for any of the outcome measures. The present results indicate that protein supplementation does not attenuate any of the indirect indices of EIMD imposed by concurrent exercise, when employing great rigour around the provision of a quality habitual diet and the provision of appropriate supplemental controls.

Does Carbohydrate Intake During Endurance Running Improve Performance? A Critical Review

Wilson, PB. Does carbohydrate intake during endurance running improve performance? A critical review. J Strength Cond Res 30(12): 3539-3559, 2016-Previous review articles assessing the effects of carbohydrate ingestion during prolonged exercise have not focused on running. Given the popularity of distance running and the widespread use of carbohydrate supplements, this article reviewed the evidence for carbohydrate ingestion during endurance running. The criteria for inclusion were (a) experimental studies reported in English language including a performance task, (b) moderate-to-high intensity exercise >60 minutes (intermittent excluded), and (c) carbohydrate ingestion (mouth rinsing excluded). Thirty studies were identified with 76 women and 505 men. Thirteen of the 17 studies comparing a carbohydrate beverage(s) with water or a placebo found a between-condition performance benefit with carbohydrate, although heterogeneity in protocols precludes clear generalizations about the expected effect sizes. Additional evidence suggests that (a) performance benefits are most likely to occur during events >2 hours, although several studies showed benefits for tasks lasting 90-120 minutes; (b) consuming carbohydrate beverages above ad libitum levels increases gastrointestinal discomfort without improving performance; (c) carbohydrate gels do not influence performance for events lasting 16-21 km; and (d) multiple saccharides may benefit events >2 hours if intake is ≥1.3 g·min Given that most participants were fasted young men, inferences regarding women, adolescents, older runners, and those competing in fed conditions are hampered. Future studies should address these limitations to further elucidate the role of carbohydrate ingestion during endurance running.

Ratings of Perceived Exertion Throughout an Ultramarathon during Carbohydrate Ingestion

Ratings of perceived exertion (RPE) and their relation to selected physiological mediators during endurance exercise have been limited to laboratory settings. The present study characterized the pattern of change in perceptual responses and examined the relation between RPE and selected physiological variables during a long competitive sporting event, i.e., an ultramarathon race (68 km). A single-group design was employed in which all of the 28 subjects provided their perceptual ratings (11.9 ± 0.2) and heart rate (HR) (138 ± 3) periodically (every 5 km) throughout the ultramarathon, and selected physiological responses were measured before, once during (32 km), and immediately after the race. Runners drank approximately 1,000 ml of carbohydrate beverage each hour (60 gm carbohydrate hr.−1) and ate 2 or 3 carbohydrate gel packs per hour (25 gm each−1). RPE increased significantly throughout the course of the ultramarathon. No significant correlations were found between RPE and HR at any time throughout the ultramarathon. RPE averaged 10.4 ± 0.4 at the beginning of the race (6.4 km) and 15.4 ± 0.4 at the conclusion of the race. Subjects maintained 76.9 ± 1.1% of maximal heart rate; however, there was a tendency for heart rate to drop significantly after 32 km. Significant time main effects were found for serum glucose, insulin, and cortisol throughout the race. However, no significant correlations were found between RPE and any of these physiological mediators. These data indicate that during an ultramarathon race there is a progressive increase in RPE without an accompanying increase in HR or decrease in blood glucose. Therefore, during competitive self-paced exercise the perceptual responses may be mediated through other neurological and physiological mechanisms.

Exercise and Regulation of Lipid Metabolism

The increased prevalence of hyperlipidemia, hypertriglyceridemia, hypercholesterolemia, and fatty liver disease has provided increasingly negative connotations toward lipids. However, it is important to remember that lipids are essential components supporting life. Lipids are a class of molecules defined by their inherent insolubility in water. In biological systems, lipids are either hydrophobic (containing only polar groups) or amphipathic (possess polar and nonpolar groups). These characteristics lend lipids to be highly diverse with a multitude of functions including hormone and membrane synthesis, involvement in numerous signaling cascades, as well as serving as a source of metabolic fuel supporting energy production. Exercise can induce changes in the lipid composition of membranes that effect fluidity and cellular function, as well as modify the cellular and circulating environment of lipids that regulate signaling cascades. The purpose of this chapter is to focus on lipid utilization as metabolic fuel in response to acute and chronic exercise training. Lipids utilized as an energy source during exercise include circulating fatty acids bound to albumin, triglycerides stored in very-low-density lipoprotein, and intramuscular triglyceride stores. Dynamic changes in these lipid pools during and after exercise are discussed, as well as key factors that may be responsible for regulating changes in fat oxidation in response to varying exercise conditions.

Cardiovascular and ride time-to-exhaustion effects of an energy drink

Background

Currently, there are few studies on the cardiovascular and fatigue effects of commercially available energy drinks. This study investigated the effects of Monster energy drink (Monster Beverage Corporation, Corona, California), on resting heart rate (HR), heart rate variability (HRV), ride time-to-exhaustion, peak exercise HR, respiratory exchange ratio (RER), and peak rating of perceived exertion (RPE).

Methods

The study used a double-blind, randomized, placebo controlled, crossover design. After an 8-hr fast, 15 subjects consumed Monster Energy Drink (ED standardized to 2.0 mg * kg^-1 caffeine) or a flavor-matched placebo preexercise. Resting HR and HRV were determined. After an initial submaximal workload for 30 minutes, subjects completed 10 min at 80% ventilatory threshold (VT) and rode until volitional fatigue at 100% VT.

Results

Resting HR was significantly different (ED: 65+/-10 bpm vs. placebo: 58+/-8 bpm, p = 0.02), but resting HRV was not different between the energy drink and placebo trials. Ride time-to-exhaustion was not significantly different between trials (ED: 45.5+/- 9.8 vs. placebo: 43.8+/-9.3 min, p = 0.62). No difference in peak RPE (ED: 9.1 +/- 0.5 vs. placebo: 9.0 +/- 0.8, p = 1.00) nor peak HR (ED: 177 +/- 11 vs. placebo: 175 +/- 12, p = 0.73) was seen. The RER at 30% of VT was significantly different (ED: 0.94 +/- 0.06 vs. placebo: 0.91 +/- 0.05, p = 0.046), but no difference between the two conditions were seen at the other intensities.

Conclusion

Although preexercise ingestion of the energy drink does increase resting HR there was no alteration in HRV parameters. Ride time-to-exhaustion was not enhanced.

Body mass changes and nutrient intake of dinghy sailors while racing

Dinghy sailing is a physically challenging sport with competitors on water for several hours. Regulations and space in the boat limit the amount of food and fluid competitors can carry. Consequently, it is possible that the hydration and nutritional status of dinghy sailors may be compromised while racing. Despite this, the food and fluid intake of sailors while racing are unknown. The aim of this study was to assess the dietary intake of a group of club sailors while racing and compare this with current sports nutrition guidelines. Thirty-five sailors (9 females, 26 males) were monitored during a club regatta. Body mass changes were measured before and after racing, as were food and fluid intake. Results showed that most participants were in negative fluid balance after racing (males: mean -2.1%[95% confidence limits -1.7 to -2.5%]; females: -0.9%[0 to -1.8%]), most likely due to low voluntary fluid intake (males: 1215 ml [734 to 1695 ml]; females: 792 ml [468 to 1117 m]). Carbohydrate intake (males: 59 g [21 to 97 g]; females: 30 g [0 to 61 g]) was below recommendations for normal sports activity. Results revealed that the nutritional practices of club sailors do not comply with current sports nutrition guidelines. However, the performance implications of a compromise in nutrient intake remain to be investigated. Practical advice on methods of overcoming space limitations for the carriage of adequate fluid and food is offered.

Muscle mechanical characteristics in fatigue and recovery from a marathon race in highly trained runners

The aim of the present study was to examine muscle mechanical characteristics before and after a marathon race. Eight elite runners underwent a pre-test 1 week before the marathon and post-tests 30 min, two and five-day-post-marathon. Actual marathon race performance was 2:34:40 +/- 0:04:13. Energy expenditure at marathon pace (EE(Mpace)) was elevated 4% post-marathon (pre: 4,465 +/- 91 vs. post 4,638 +/- 91 J kg bodyweight(-1) km(-1), P < 0.05), but was lowered by 6 and 9.5% two- and five-day-post-marathon compared to EE(Mpace) pre-marathon. Countermovement jump (CMJ) power decreased 13% post-marathon (pre: 21.5 +/- 0.9 vs. post: 18.9 +/- 1.2 W kg(-1); P < 0.05) and remained depressed two- (18%) and five-day (12%) post-marathon. CMJ force was unaltered across all four tests occasions. Knee extensor and plantar flexor maximal voluntary contraction (MVC) decreased from 176.6 +/- 9.5 to 136.7 +/- 16.8 Nm and 144.9 +/- 8.7 to 119.2 +/- 15.1 Nm post-marathon corresponding to 22 and 17%, respectively (P < 0.05). No significant changes were detected in evoked contractile parameters, except a 25% increase in force at 5 Hz, and low frequency fatigue was not observed. In conclusion, leg muscle power decreased acutely post-marathon race and recovered very slowly. The post-marathon increase in EE(Mpace) might be attributed to a reduction in stretch shortening cycle efficiency. Finally, since MVC was reduced after the marathon race without any marked changes in evoked muscle contractile properties, the strength fatigue experienced by the subjects in this study seems to be related to central rather than peripheral mechanisms.

Alterations of Neuromuscular Function After Prolonged Running, Cycling and Skiing Exercises

It is well known that impairment of performance resulting from muscle fatigue differs according to the types of contraction involved, the muscular groups tested and the exercise duration/intensity. Depending on these variables, strength loss with fatigue can originate from several sites from the motor cortex through to contractile elements. This has been termed 'task dependency of muscle fatigue'. Only recently have studies focused on the origin of muscle fatigue after prolonged exercise lasting 30 minutes to several hours. Central fatigue has been shown to contribute to muscle fatigue during long-distance running by using different methods such as the twitch interpolation technique, the ratio of the electromyogram (EMG) signal during maximal voluntary contraction normalised to the M-wave amplitude or the comparison of the forces achieved with voluntary- and electrically-evoked contractions. Some central activation deficit has also been observed for knee extensor muscles in cycling but central fatigue after activities inducing low muscular damage was attenuated compared with running. While supraspinal fatigue cannot be ruled out, it can be suggested that spinal adaptation, such as inhibition from type III and IV group afferents or disfacilitation from muscle spindles, contributes to the reduced neural drive after prolonged exercise. It has been shown that after a 30 km run, individuals with the greatest knee extensor muscle strength loss experienced a significant activation deficit. However, central fatigue alone cannot explain the entire strength loss after prolonged exercise. Alterations of neuromuscular propagation, excitation-contraction coupling failure and modifications of the intrinsic capability of force production may also be involved. Electrically-evoked contractions and associated EMG can help to characterise peripheral fatigue. The purpose of this review is to further examine the central and peripheral mechanisms contributing to strength loss after prolonged running, cycling and skiing exercises.

Effect of carbohydrate ingestion on ratings of perceived exertion during a marathon

PURPOSE

The purpose of this study was to investigate the effects of carbohydrate substrate availability on ratings of perceived exertion (RPE) and hormonal regulation during a competitive marathon.

METHODS

A randomized, double-blind study design was used in which subjects ran the marathon, and every 3.2 km, RPE and heart rate were measured. The marathoners were randomly assigned to receive carbohydrate (C) (N = 48) or placebo (P) (N = 50) beverages at a rate of 1 L x h(-1) during the race.

RESULTS

Heart rate (%(HRMAX) ) was lower in P (82.0% +/- 0.6) than C (84.2% +/- 0.6) (P < 0.01), especially during the final 10 km: (78.7% +/- 1.0) and (84.5% +/- 0.7), respectively (P < 0.001). RPE was not significantly different between P and C throughout the marathon (P = 0.08) or during the final 10 km: (16.8 +/- 0.3) and (16.1 +/- 0.3), respectively (P = 0.06). Postrace plasma glucose (P < 0.001), insulin (P < 0.001), and lactate (P < 0.05) levels were significantly lower in P than C, and postrace cortisol (P < 0.05) significantly higher in P compared with C.

CONCLUSIONS

Marathoners ingesting carbohydrate compared with placebo beverages were able to run at a higher intensity while reporting a nonsignificant difference in RPEs during a competitive race.

Changes in oxygen consumption during and after a downhill run in masters long-distance runners

OBJECTIVE

To determine whether oxygen consumption during submaximal running increases in proportion to years of accumulated training and racing in masters runners after a bout of downhill running.

SETTING

University of Cape Town, Sports Science Institute of South Africa.

PARTICIPANTS

Seventeen male masters distance runners (45-55 years) with a range of training (3,536 km to 79,320 km) and racing (205 km to 12,218 km) experience.

INTERVENTION

A 40-minute continuous treadmill run, at 70% of peak treadmill running speed, consisting of two horizontal runs of 10 minutes each, separated by a 20-minute downhill (-10%) run.

MAIN OUTCOME MEASURES

Heart rate and oxygen consumption were measured continuously during the run. Data were analyzed to identify correlations between the end of the first horizontal section (minute 10) and the first minute of the second horizontal run (minute 31). Delta values were related to current training mileage (km/wk), total accumulated racing distance (km), and total accumulated training distance (km).

RESULTS

There were significant changes in both heart rate (p < 0.001) and oxygen consumption (p < 0.001) over time during the 40-minute run. There were no significant relationships between the change in oxygen consumption (delta) between minute 10 and minute 31 and total accumulated training mileage, total accumulated racing mileage, and current training.

CONCLUSIONS

The results of this study suggest either that submaximal oxygen consumption is not a sensitive marker of changes in neuromuscular activity or that the downhill protocol did not impose a sufficient eccentric stress for the subjects.

Alterations of neuromuscular function after an ultramarathon

Neuromuscular fatigue of the knee extensor (KE) and plantar flexor (PF) muscles was characterized after a 65-km ultramarathon race in nine well-trained runners by stimulating the femoral and tibial nerves, respectively. One week before and immediately after the ultramarathon, maximal twitches were elicited from the relaxed KE and PF. Electrically evoked superimposed twitches of the KE were also elicited during maximal voluntary contractions (MVCs) to determine maximal voluntary activation. MVC and maximal voluntary activation decreased significantly after the ultramarathon (-30.2 +/- 18.0% and -27.7 +/- 13.0%, respectively; P < 0.001). Surprisingly, peak twitch increased after the ultramarathon from 15.8 +/- 6.3 to 19.7 +/- 3.3 N. m for PF (P < 0.01) and from 131.9 +/- 21.2 to 157.1 +/- 35.9 N for KE (P < 0.05). Also, shorter contraction and half-relaxation times were observed for both muscles. The compound muscle action potentials (M wave) were not significantly altered by the ultramarathon with the exception of the soleus, which showed a slightly higher M-wave amplitude after the running. From these results, it can be concluded that 65 km of running 1) severely depressed the maximal voluntary force capacity mainly because of a decrease in maximal voluntary activation, 2) potentiated the twitch mechanical response, and 3) did not change significantly the M-wave characteristics.

Maximal oxygen uptake: "classical" versus "contemporary" viewpoints: a rebuttal

Bassett and Howley contend that the 1996 J. B. Wolffe lecture is erroneous because: 1) A. V. Hill did establish the existence of the "plateau phenomenon," 2) the maximum oxygen consumption (VO2max) is limited by the development of anaerobiosis in the active muscle, and 3) endurance performance is also determined by skeletal muscle anaerobiosis because the VO2max is the best predictor of athletic ability. As a result, 4) cardiovascular and not skeletal muscle factors determine endurance performance. They further contend that Hill's "scientific hunches were correct," requiring "only relatively minor refinements" in the past 70 yr. But the evidence presented in this rebuttal shows that Hill neither sought nor believed in either the "plateau phenomenon" or the concept of the individual maximum oxygen consumption. These twin concepts were created by Taylor et al. (97) in 1955 and erroneously attributed to Hill. Rather Hill believed that there was a universal human VO2max of 4 L x min(-1). His error resulted from his incorrect belief that the real VO2 unmeasurable because it includes a large "anaerobic component," rose exponentially at running speeds greater than 13.2 km x h(-1). But Hill and his colleagues were indeed the first to realize the danger that a plateau in cardiac output (CO) and hence in VO2 would pose for the heart itself. For unlike skeletal muscle, the pumping capacity of the heart is both dependent on, but also the determinant of, its own blood supply. Thus, if the CO reaches a peak causing the "plateau phenomenon," the immediate cause of that peak will have been a plateau in myocardial oxygen delivery, causing a developing myocardial ischemia. The ischemia must worsen as exercise continues beyond the supposed VO2 "plateau." To accommodate this dilemma, Hill and his colleagues proposed a governor "either in the heart muscle or in the nervous system" necessary to prevent myocardial ischemia developing during maximal exercise. This governor would cause maximal exercise to terminate before the development of a plateau in either coronary flow, CO, or VO2, or the onset of skeletal muscle anaerobiosis. Accordingly, a new physiological model is proposed in which skeletal muscle recruitment is regulated by a central "governor" specifically to prevent the development of a progressive myocardial ischemia that would precede the development of skeletal muscle anaerobiosis during maximum exercise. As a result cardiovascular function "limits" maximum exercise capacity, probably as a result of a limiting myocardial oxygen delivery. The model is compatible with all the published findings of cardiovascular function during exercise in hypobaric hypoxia, in which there is a greater likelihood that myocardial hypoxia will develop.

Effect of meal frequency and timing on physical performance

Two areas of sports nutrition in which the periodicity of eating has been studied relate to: (1) the habitually high energy intakes of many athletes, and (2) the optimization of carbohydrate (CHO) availability to enhance performance. The present paper examines how the timing and frequency of food and fluid intake can assist the athlete and physically-active person to improve their exercise performance in these areas. Frequent eating occasions provide a practical strategy allowing athletes to increase energy intake while concomitantly reducing the gastric discomfort of infrequent large meals. The optimization of CHO stores is a special challenge for athletes undertaking prolonged training or competition sessions. This is a cyclical process with post-exercise CHO ingestion promoting muscle and liver glycogen re-synthesis; pre-exercise feedings being practised to optimize substrate availability and feedings during exercise providing a readily-available source of exogenous fuel as endogenous stores become depleted. The timing and frequency of CHO intake at these various stages are crucial determinants for optimizing fuel availability to enhance exercise capacity.

Fuel kinetics during intense running and cycling when fed carbohydrate

On two occasions, six well-trained, male competitive triathletes performed, in random order, two experimental trials consisting of either a timed ride to exhaustion on a cycle ergometer or a run to exhaustion on a motor-driven treadmill at 80% of their respective peak cycling and peak running oxygen (VO2 max) uptakes. At the start of exercise, subjects drank 250 ml of a 15 g.100 ml-1 w/v [U-14C]glucose solution and, thereafter, 150 ml of the same solution every 15 min. Despite identical metabolic rates [VO2 3.51 (0.06) vs 3.51 (0.10) 1.min-1; values are mean (SEM) for the cycling and running trials, respectively], exercise times to exhaustion were significantly longer during cycling than running [96 (14) vs 63 (11) min; P < 0.05]. The superior cycling than running endurance was not associated with any differences in either the rate of blood glucose oxidation [3.8 (0.1) vs 3.9 (0.4) mmol.min-1], or the rate of ingested glucose oxidation [2.0 (0.1) vs 1.7 (0.2) mmol.min-1] at the last common time point (40 min) before exhaustion, despite higher blood glucose concentrations at exhaustion during running than cycling [7.0 (0.9) vs 5.8 (0.5) mmol.l-1; P < 0.05]. However, the final rate of total carbohydrate (CHO) oxidation was significantly greater during cycling than running [24.0 (0.8) vs 21.7 (1.4) mmol C6.min-1; P < 0.01]. At exhaustion, the estimated contribution to energy production from muscle glycogen had declined to similar extents in both cycling and running [68 (3) vs 65 (5)%]. These differences between the rates of total CHO oxidation and blood glucose oxidation suggest that the direct and/or indirect (via lactate) oxidation of muscle glycogen was greater in cycling than running.

Exercise and the oxidation and storage of glucose, maize-syrup solids and sucrose determined from breath 13CO2

In order to determine which of maize syrup solids, glucose and sucrose were more readily oxidised during exercise and least readily oxidised afterwards, the rates of oxidation of three almost identical isoenergetic solutions of carbohydrates (330 ml of 18.5% w/v solutions of glucose, maize syrup solids and sucrose, 989-1050 kJ total energy) naturally enriched with 13C were examined at rest and during and after 1 h uphill walking at 75% maximum oxygen uptake (VO2max) in nine subjects [mean (SEM) VO2max, 45.4 (0.9) ml.kg-1.min-1]. Rates of production of expired 13CO2 were used to estimate rates of oxidation of each exogenous substrate. Energy expenditure and the contributions from total carbohydrate and fat oxidation were calculated from whole-body gas exchange. At rest, maize syrup solids were oxidised less than sucrose during the 1st h [glucose 2.7 (0.2) g.h-1, maize syrup solids 1.9 (0.3) g.h-1, sucrose 3.7 (0.2) g.h-1; maize syrup solids vs sucrose P < 0.01], but this difference disappeared after a further 3 h at rest [glucose 8.3 (0.5) g.h-1, maize syrup solids 7.7 (0.5) g.h-1, sucrose 8.1 (0.4) g.h-1]. During exercise, all the carbohydrates were oxidised to the same extent [glucose 23.0 (2.8) g.h-1, maize syrup solids 23.9 (3.4) g.h-1, sucrose 27.5 (2.6) g.h-1) but during 4 h of recovery after exercise, maize syrup solids were oxidised least [glucose 4.6 (0.1) g.h-1, maize syrup solids 3.7 (0.1) g.h-1, sucrose 6.4 (0.1) g.h-1; P < 0.05] suggesting that it may be stored to a greater extent. The results suggest that 18.5% glucose, maize syrup solids and sucrose solutions were equally well oxidised during exercise. During recovery from exercise maize syrup solids were oxidised less than glucose, which in turn was oxidised less than sucrose.

Effect of carbohydrate ingestion subsequent to carbohydrate supercompensation on endurance performance

This investigation determined whether carbohydrate ingestion during prolonged moderate-intensity exercise enhanced endurance performance when the exercise was preceded by carbohydrate supercompensation. Seven male trained cyclists performed two trials at an initial power output corresponding to 71 +/- 1% of their peak oxygen consumption. During the trials, subjects ingested either a 6% glucose/sucrose (C) solution or an equal volume of artificially flavored and sweetened placebo (P) every 20 min throughout exercise. Both C and P were preceded by a 6-day carbohydrate supercompensation procedure in which subjects undertook a depletion-taper exercise sequence in conjunction with a moderate- and high-carbohydrate diet regimen. Statistical analysis of time to exhaustion, plasma glucose concentration, carbohydrate oxidation rate, fat oxidation rate, and plasma glycerol concentration indicated that in spite of a carbohydrate supercompensation procedure administered prior to exercise, carbohydrate ingestion during exercise can exert an additional ergogenic effect by preventing a decline in blood glucose levels and maintaining carbohydrate oxidation during the later stages of moderate-intensity exercise.

Failure of low dose carbohydrate feeding to attenuate glucoregulatory hormone responses and improve endurance performance

The effects of ingesting a low dose of CHO on plasma glucose, glucoregulatory hormone responses, and performance during prolonged cycling were investigated. Nine male subjects cycled for 165 min at approximately 67% peak VO2 followed by a two-stage performance ride to exhaustion on two occasions in the laboratory. Every 20 min during exercise, subjects consumed either a flavored water placebo (P) or a dilute carbohydrate beverage (C). Blood samples were collected immediately before, every 20 min throughout, and immediately after exercise. Plasma was analyzed for glucose, lactate, free fatty acids (FFA), and various glucoregulatory hormones. VO2, RER, heart rate, perceived exertion, and exercise performance were also measured. Lactate, FFA, epinephrine, norepinephrine, ACTH, cortisol, and glucagon increased with exercise whereas glucose and insulin decreased (p < or = .05). Except for a small difference in glucose at 158 min of exercise and at exhaustion, no significant differences were found between drinks for any of the variables studied (P > or = .05). Ingestion of 13 g carbohydrate per hour is not sufficient to maintain plasma glucose, attenuate the glucoregulatory hormone response, and improve performance during prolonged moderate intensity cycling.

Timing and method of increased carbohydrate intake to cope with heavy training, competition and recovery

Based upon the fact that fatigue during intense prolonged exercise is commonly due to depletion of muscle and liver glycogen which limits both training and competitive performance, this paper has proposed extraordinary dietary practices which generally advocate high carbohydrate intake at all times before, during and after exercise. The simple goal is to have as much carbohydrate in the body as possible during the latter stages of prolonged intense exercise when the ability for intense exercise usually becomes limiting to performance. This theory is put into practice by recommending that carbohydrate intake after exhaustive exercise should average 50 g per 2 h of mostly moderate and high glycaemic carbohydrate foods. The aim should be to ingest a total of about 600 g in 24 h. Carbohydrate intake should not be avoided during the 4 h period before exercise and in fact it is best to eat at least 200 g during this time. When possible, carbohydrate should be ingested during exercise, generally in the form of solutions containing glucose/sucrose/maltodextrins, at a rate of 30-60 g h-1. Emphasis has been placed upon eating the optimal amount and best type of carbohydrate at the proper times because these practices demand a large amount of food. When diet is not carefully planned according to these guidelines, endurance athletes tend to consume too little carbohydrate because they become satiated with high fat in their diet and they go through periods in the day when recovery of glycogen stores is suboptimal and thus precious time is wasted.

Effect of hyperglycaemia on muscle glycogen mobilization during muscle contractions in the rat

In the rat, muscle glycogen is mobilized during the first stage of exercise, despite normoglycaemia. The aim of the present study was to examine if this process could be prevented or reduced by hyperglycaemia. Three experiments were carried out: in the first, rats were forced to run on a treadmill; in the second the gastrocnemius muscle group was made to contract by stimulation of the sciatic nerve and in the third adrenaline was administered subcutaneously. Each group was divided into two subgroups: control and enriched with glucose (hyperglycaemic). It was shown that hyperglycaemia has no effect on running-induced glycogen mobilization in hind-limb muscles of different fibre composition but prevented it totally in diaphragm muscle. Hyperglycaemia also did not affect the glycogen mobilization induced by stimulation of the sciatic nerve. However, it delayed and reduced markedly the glycogenolytic effect of adrenaline. It is concluded that increased glycogenolysis in muscles at the beginning of exercise may be a consequence of a delay in the activation of glucose transporting mechanisms in muscle cells.

Lipid depletion and repletion in skeletal muscle following a marathon

Intramuscular lipid content was investigated in muscle biopsies from 10 well-trained endurance athletes before, immediately after, and 1, 3, 5, and 7 days after a marathon. Diets were controlled throughout the entire period of the study. Triglyceride content was ultrastructurally determined by the use of stereological methods. The volume percent lipid significantly decreased after the marathon and was lowest at 3 days post-marathon, rising slightly but still 35% lower than the pre-marathon value by 7 days post-marathon. Glycogen granules were abundant and tightly packed in the pre-marathon biopsies, scarce immediately post-marathon, and abundant, but less tightly packed, 7 days post-marathon. Post-marathon fluctuations in the volume percentages of mitochondria indicated possible fluid shifts within the muscle fibers: dehydration immediately post-marathon followed by rehydration with possible edema. Assuming the content of mitochondria remained constant throughout the recovery period, the ratio of the volume percentage of lipid to the volume percentage of mitochondria indicated that lipid content may have reached pre-marathon levels after 7 days post-marathon.

Orienteering performance and ingestion of glucose and glucose polymers

The benefit of glucose polymer ingestion in addition to 2.5 per cent glucose before and during a prolonged orienteering competition was studied. The final time in the competition in the group ingesting 2.5 per cent glucose (group G, n = 10) was 113 min 37 s +/- 8 min 11 s, and in the group which had additionally ingested glucose polymer (group G + GP, n = 8) 107 min 18s +/- 4 min 41 s (NS). One fifth (21 per cent) of the time difference between the two groups was due to difference in orienteering errors. Group G + GP orienteered the last third of the competition faster than group G (p less than 0.05). The time ratio between the last third of the competition and the first third of the competition was lower in group G + GP than in group G (p less than 0.05). After the competition, there was statistically insignificant tendency to higher serum glucose and lower serum free fatty acid concentrations in group G + GP, and serum insulin concentration was higher in group G + GP than in group G (p less than 0.05). Three subjects reported that they exhausted during the competition. These same three subjects had the lowest serum glucose concentrations after the competition (2.9 mmol.1(-1), 2.9 mmol.1(-1), 3.5 mmol.1(-1] and all of them were from group G. It is concluded that glucose polymer syrup ingestion is beneficial for prolonged psychophysical performance.

Glucose polymer syrup attenuates prolonged endurance exercise-induced vasopressin release

We investigated the effect of glucose and glucose polymer ingestion on plasma arginine vasopressin (pAVP) levels, on plasma osmolality (p-osm), and on performance during two prolonged endurance events. The study subjects were 37 Finnish elite endurance athletes, of whom 18 were orienteers and 19 cross-country skiers. Plasma AVP increased in both combined glucose and glucose polymer groups, but the increase in the glucose polymer group was significantly smaller (P less than 0.001) than that in the glucose group. A significant change in p-osm caused a significant change in pAVP and vice versa. Both the orienteers and the skiers on glucose polymer tended to have more success in the competition; the orienteers on glucose polymer ran the last third of the competition significantly faster than those on glucose (P less than 0.05). It is suggested, in the light of the smaller pAVP response, that after glucose polymer ingestion the physical stress in prolonged endurance exercise is smaller than after ingestion of glucose.

Factors Associated With Collapse During and After Ultramarathon Footraces: A Preliminary Study

In brief: In a preliminary study to evaluate possible causes of exhaustion, 48 runners who collapsed during or after an ultramarathon race were questioned about their training methods. The training profiles were compared with those of matched controls who had finished the same race without collapsing. Possible contributing factors found among the collapsed runners included inadequate training, failing to carbohydrate load, not eating a prerace breakfast, prerace illness, and hypothermia (during one race, which was run in cold, wet, windy conditions). The authors suggest that regardless of athletic ability or previous racing experience, runners may be prone to collapse during or after ultramarathon races if their prerace training and preparation are inadequate.

The danger of an inadequate water intake during prolonged exercise. A novel concept re-visited

To prevent thermal injuries during distance running, the American College of Sports Medicine proposes that between 0.83 and 1.65 l of water should be ingested each hour during prolonged exercise. Yet such high rates of fluid intake have been reported to cause water intoxication. To establish the freely-chosen rates of fluid intake during prolonged competitive exercise, we measured fluid intake during, body weight before and after, and rectal temperature after competition in a total of 102 runners and 91 canoeists competing in events lasting from 170-340 min. Fluid intakes during competition ranged from 0.29-0.62 l.h-1; rates of water loss ranged from 0.69-1.27 l.h-1 in the runners; values were lower in the canoeists. Mean post-race rectal temperatures ranged from 38.0-39.0 degrees C. There was no relationship between the degree of dehydration and post-race rectal temperature. We conclude that hyperthermia is uncommon in prolonged competitive events held in mild environmental conditions, and that exercise intensity, not the level of dehydration, is probably the most important factor determining the postexercise rectal temperature. During prolonged exercise in mild environmental conditions, a fluid intake of 0.5 l.h-1 will prevent significant dehydration in the majority of athletes.